gcd(a,b)=gcd(a,b−a)
gcd(a,b,c)=gcd(a,b−a,c−b)
gcd(a1,a2,⋯,an)=gcd(a1,a2−a1,a3−a2,⋯,an−an−1)
int c = _gcd(a,b);
gcd(a,b)=gcd(a,b−a)
gcd(a,b,c)=gcd(a,b−a,c−b)
gcd(a1,a2,⋯,an)=gcd(a1,a2−a1,a3−a2,⋯,an−an−1)
int c = _gcd(a,b);